This invention relates to a method for reducing the accumulation of radioactive substances on the piping inside surfaces of primary cooling water system in contact with reactor water in a newly constructed nuclear power plant and provides a reactor water pH control system suitable for adjusting the pH of reactor water to a slightly alkaline side when an oxide film having a high effect on suppressing deposition of radioactive substances is to be formed. Pipings, pumps, valves, etc. used in the primary cooling water system in nuclear power plants are made of stainless steel, carbon steel, sterite, etc., which will be hereinafter referred to as "out-of-core components". These metals as component materials start to undergo corrosion little by little from the time of start-up of the plant operation, and the constituent metal elements dissolve into the primary cooling water. In a boiling water type, nuclear power plant these corrosion products are removed in a condensate water purification unit, but the corrosion products that cannot be removed in the purification unit and other corrosion products formed in the feedwater system after the condensate water purification unit accumulate in the reactor of the plant.
Most of the corrosion products brought into the reactor turn into oxides, deposit onto fuel rods and are subjected to neutron irradiation. As a result, radioactive nuclides such as .sup.60 Co, .sup.58 Co, .sup.51 Cr, .sup.54 Mn, etc. are formed. These radioactive nuclides are redissolved into the reactor water and turn into ions or suspended as insoluble solids, which will be hereinafter referred to as crud. A portion of the ions and crud are removed in a purification unit of a reactor water clean-up system for purifying reactor water; whereas the remainder circulates through the primary cooling water system of reactor and deposits onto the surfaces of out-of-core components. Thus, the dose rate increases on the surfaces of out-of-core components and radiation exposure of plant personnel during the periodic inspection time is a problem.
In order to reduce the amount of deposited radioactive substances, methods for suppressing formation of the corrosion products as a source have been proposed, wherein corrosion of component material is suppressed, for example, by using corrosion-resistant materials or by injecting oxygen into the feed water. However, the corrosion of the component material in the feed water system cannot be thoroughly suppressed even according to these methods, and the concentration of radioactive substances in reactor water is not thoroughly reduced. Thus, the increase in the dose rate due to the deposition of radioactive substances onto the surfaces of the out-of-core components still remains as a problem.
In order to solve the problem, methods for removing the radioactive substances deposited on the surfaces of the out-of-core components have been proposed. According to one method for removing the radioactive substances from the surfaces of the out-of-core components in complicated structures in the reactor system, oxide films formed on the surfaces of the out-of-core components are dissolved with a chemical to remove the radioactive substances contained in the films. The method still has such problems that, even though the dose rate can be reduced tentatively, the out-of-core component materials will be again exposed to reactor water containing radioactive substances at a high concentration when the operation of the plant is restarted, and a new oxide film will be formed again thereby to undergo rapid recontamination.
To overcome the problems, methods for forming an oxide film before the startup of a plant or at the initial period of plant operation, where there are no radioactive substances or the concentrations of radioactive substances are very low in the reactor water, thereby suppressing the deposition of radioactive substances, are disclosed in Japanese Patent Applications Kokai (Laid-open) Nos. 59-12390 and 59-37498. The disclosure is summarized as follows.
Radioactive nuclides dissolved in reactor water are taken into the oxide film formed on the surface of stainless steel as out-of-core component by corrosion of the stainless steel in the course of its formation. In high temperature water, the oxide film grows mainly inwardly (toward the matrix metal side) at the boundary surface between the film and the matrix metal, and the radioactive nuclides are diffused and migrate inwardly through the film and taken into the thus formed oxide film at the same boundary surface. Thus, in order to suppress the deposition of radioactive substances, diffusion of the radioactive substances through the oxide film must be suppressed. Furthermore, the deposition rate of radioactive substances has been found to be in a correlation with the film growth rate, and thus the suppression of film growth leads to the reduction in the deposition of radioactive substances. That is, the correlation of the deposition rate of radioactive substances to the film growth rate is given by taking the radioactive substances into the oxide film at the film growth points. Thus, the suppression of film growth can correspondingly reduce the frequency of taking the radioactive substances into the film. That is, it is suppressed to take the radioactive substances into the film.
Increase in the film growth (m) on the stainless steel in reactor water is in a logarithmic relation to time (t) as given by the following equation (1): EQU m=K.sub.1 log (K.sub.2 t+1) (1)
where K.sub.1 and K.sub.2 are constants.
That is, the film growth rate decreases with increasing film growth with time. Thus, if a non-radioactive film is formed in advance on the stainless steel, the quantity of film to be newly formed can be reduced, and the quantity of radioactive substances to be taken into the newly formed oxide film can be suppressed. In other words, the treatment for suppressing the oxide film formation is nothing but a treatment for suppressing the corrosion of out-of-core components and also serves to reduce the quantity of soluble components from the core component materials. Thus, the treatment for forming an oxide film in a non-radioactive circumstance in advance can reduce the quantity of radioactive substances to be dissolved from the core component materials which are to be directly radioactivated in the reactor water, and can also contribute to the reduction in the concentration of radioactive substances in the reactor water.
However, the deposition behavior of radioactive substances considerably depends upon the properties of an oxide film to be formed in advance. For example, the behavior of radioactive ions depends upon the electrically charged state of an oxide film formed in advance. Furthermore, the growth rate of an oxide film to be newly formed on the surfaces of the out-of-core components after the out-of-core components are brought in contact with reactor water containing the radioactive substances depends upon the properties of the existing oxide film. Thus, it is necessary to conduct the oxidation treatment of out-of-core components according to the most appropriate method under conditions of reactor water circumstance. Furthermore in an actual plant it is necessary to conduct the oxidation treatment within the restriction of water quality control standard and operating control standard without impairing the plant. To this end, it is necessary to make appropriate selection of the time for conducting the oxidation treatment and the method for alkali injection from the plant construction period and the startup test period and also to make appropriate selection of plant operating procedure at the oxidation treatment.